1. Technical Field
The present invention relates to a control apparatus for a hybrid electric vehicle, and particularly to a control apparatus that maximizes vehicle operating efficiency by determining a vehicle maximum operating point for all engine operating conditions.
2. Discussion
A conventional hybrid electric vehicle includes an engine, two electric motors, and a battery (hereinafter also termed a two-motor type hybrid electric vehicle), such as is disclosed in Japanese Patent Application No. Hei. 8-239380. In such a vehicle, a controller determines a vehicle drive-power requirement value defined by vehicle speed (crank-angle speed) and vehicle required torque. An engine-power requirement value is calculated from both the sum of this value and a charge and discharge power requirement value. Also, an engine operating point whereat the engine is operable at high efficiency is determined, and throttle control is performed so that the engine operates at this point. The controller then controls engine speed via the first motor and controls excessive or insufficient torque of the vehicle via the second motor.
In the above conventional control apparatus, by increasing or decreasing vehicle torque by torque-addition exchange via the -motor pair, control (also termed engine efficiency-improvement control) is performed to reach high-efficiency engine operation while satisfying vehicle power requirements, i.e., the vehicle drive-torque requirement value at actual vehicle speed. Therefore, vehicle fuel consumption is improved.
However, regarding the above control application, fuel consumption tends to worsen during certain vehicle operating conditions, especially where the vehicle operating point is in a high-speed, low-torque region, irrespective of the above-described engine efficiency-improvement control. In response, a considerable portion of vehicle drive power must be passed through the motor of the motive-power transmitter, and torque conversion must be performed by electrical-power transformation. As a result thereof, even when the optimal engine operating point with respect to the engine-power requirement value is selected, the overall efficiency of the hybrid-system deviates from the optimal point thereof due to the losses due to the motors. That is to say, the overall efficiency of the hybrid system decreases as the conventional hybrid engine control establishing the engine operating point does not account for the motor loss.